Bonded laminated magnetic material



J. G. FORD BONDED LAMINATED MAGNETIC MATERIAL /Cjg j.

/Pf/.na/y 1 ecana/a Filed April 26, 1940 March 20, 1945.

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Patented Mar. 20, 1945 UNITED STATES PATENT OFFICE inghouse Electric d:Manufacturing Com Diny East Pittsburgh, Pa., a corporation oi Pennsyl-Appllcation April 28, 1940, Serial No. 331,785 4 Claims. (Cl. 175-21)'I'his invention relates to bonding. particularly bonding laminations ofmagnetic material in electrical machinery.

Magnetic material in laminated or sheet form is extensively utilized inthe manufacture of a wide variety of electrical apparatus, notablytransformers, dynamo-electric machines and other comparable equipment inwhich substantial quantities of magnetic ilux must be linked withelectrical conductors. To reduce the size, weight and cost of suchequipment, the physical dimensions and losses in the magnetic circuitparts which serve to conduct this flux should be kept as low aspossible.

The object of this invention is to provide for bonding cores of magneticmaterial.

A further object of the invention is to provide a composition forbonding cores of magnetic material.

A more speciiic obiect of my invention is to provide for bonding ofcores of magnetic material whereby a more eilicient electrical apparatusis effected.

The invention accordingly comprises the features of construction,combinations of elements and arrangement of parts which will beexempliiied in the construction hereinafter set forth and the scope ofthe application of which will be indicated in the claims.

Por a fuller understanding of the nature and objects of the invention,`reference should be had to the following detailed description taken inconnection with the accompanying drawing, in which Figure 1 is anelevational view of a shell-type transformer, and

Fig. 2 is an elevational view of a modiiied form of transformer.

Magnetic material suitable for use in constructing magnetic circuitparts in electrical machinery comprises silicon steel in which thesilicon content ranges from 1/2% to 5% and commonly comes in sheets of athickness of the order of 0.014 inch. Ordinary silicon steel magneticmaterial is hot rolled and its price is such that large quantities areused in the building of electrical machinery.

The magnetic properties of hot rolled silicon steel are exceeded byspecially processed silicon sheet steel. Magnetic material sold underthe trade name of Hipersil is a specially processed silicon steel whichhas better magnetic properties than the hot rolled material.

This specially processed silicon steel is cold rolled and annealed undersuch conditions that the grains in the sheet of steel are so orientedthat the direction of easiest magnetization of the individual grains orcrystals coincides with the direction of magnetization of the sheet. Ithas been found that iron, and other magnetic materials having a similarcrystalline structure, have grains or crystals which are more easilymagmetized along certain given directions. Magnetic material processedto produce a crystal arrangement which gives the sheet of magneticmaterial substantially the characteristics of the single crystal withregard to ease of magnetization may be said to have a preferredorientation.

The permeability of magnetic steel having preferred orientation isunusually high at a flux density of 16,000 lines per square centimeter.This is an improvement over the performance of hot rolled silicon steelof the same composition.

At all flux densities the steel having preferred orientation has lowerwatt losses than hot rolled silicon steel.

'l'he processes n to produce the high permeability silicon steel entailan increase in the price for this material. It is accordingly desirable,if possible, to employ the improved silicon steel for only a part of themagnetic circuit where its properties may be taken advantage of andcombine with it the less expensive hot rolled silicon steel for theremainder of the magnetic circuit.

A magnetic core design yielding the highest eiiiciency with thepreferred crystal orientation steel is desirable.

It is a purpose of this invention to provide an improved constructioninvolving the use of a novel lamination bonding composition.

Referring to the iigures of the drawing, Fig. 1 shows a shell typetransformer comprising a stack of laminations It about which the primarycoil 2t and the secondary coil 28 are wound. Abutting the upper andlower ends of the stack Il are transverse stacksof laminations i2, Il,It and 20 continuing the magnetic iiux path. The external ends of thestacks I! and il are connected by a stack of laminations 24 while thecorresponding ends oi' the stacks 20 and Il are connected by the stackof laminations 22.

It will be readily seen that it will be economical to make the centerleg or stack of laminations Il of the smallest possible cross-sectioncapable of carrying the rated magnetic flux. By using silicon steelhaving the preferred grain orientation the center leg may be worked at ahigher ilux density than the other legs of the transformer and lesscopper will be necessary. It is accordingly advantageous to employ forthe center stack i6 laminations made of Hipersil or some other highlyefficient magnetic material. The stacks i2, I4, I8, 20, 24 and 22 may bemade of sheets of ordinary hot rolled magnetic steel with satisfactoryresults. Thus without constructing the entire transformer core of themore expensive magnetic material, a satisfactory construction with amore economical arrangement of materials has been obtained. It has beenfound that the condition of th faces between the several stacks oflaminations may introduce considerable loss into the operation of theapparatus. nations are commonly stamped or cut from large sheets ofmaterial. These operations introduce Corresponding acetal resins may beused as a bonding composition. The reaction products between partiallyhydrolyzed vinyl esters such as the acetates, propionates or butyratesor mixtures thereof and an aldehyde, such as formaldehyde,propionaldehyde and butaldehyde, may, ln some instances, be used in asimilar manner for bonding chosen will depend on the requirements to bemet,

lsuch as oil resistance, softening point and bond- For example, thelamiburrs which may contact adjacent sheets and connect the twoelectrically, 'whereby eddy currents may flow from one to the other andcause large core losses.

Furthermore, the irregular surfaces created at the cut or punchedportions of the sheet do not provide for close joints between stacks oflaminations. A large air gap between stacks is a source of energy loss.to eliminate the burrs and create close joints between contiguous stacksof laminations by rst grinding the assembled stacks at the surfaces,

ground face with a chemical reagent which will eat away any fine burrsand other projections.

Coinciding with the treatment of the joints, it has been long recognizedthat laminatlons carrying a magnetic flux must be electrically insulatedfrom each other in order to decrease eddy cur'- rents. The thickness ofthe insulating medium or substance between laminations determines theelectrical resistivity to eddy current losses. On the other hand, agreat amount of insulating medium between laminations increases thephysical dimensions for a stack of laminations having a given amount ofmagnetic material. Consequently the size may increase to a point wherethe benefit of the added insulation is more than overcome by theincreased physical and cost considerations.

It is a purpose of the invention to employ a heat treated resinousadhesive composition to bond laminations into stacks suitable for coresin electrical equipment, the composition having good electricalinsulating qualities in thin layers. Other demands upon the bondingcomposition which are met, are that it is both tough and elastic inorder that the stresses encountered on heating and cooling and inordinary assembling shall not cause delamination. Another desirableproperty present in such a bonding composition is adhesion to themagnetic material. Due to the grinding and etching operations at thejoints be'- tween various stacks of laminations, the bonding compositionalso will-prevent the penetration ofwater and acid in order that onlythe end faces of the sheet material be etched and not the flat faces.

A resin comprising polyvinyl acetal, which is the reaction product of'70%l hydrolyzed vinyl acetate and acetaldehyde, and which has a. 'Isecond viscosity for a solution by theV nitrocellulose test method, hasbeen found to be a satisfactory base for the bonding composition. Thisresin has a long chain molecule structure which gives good' adhesiveproperties. For high temperatures in the electrical machinery, forexample, in a range of 90 C. to 120 C., this resin may be used alone forbonding laminations.

It is accordingly proposed ing strength.

The polyvinyl acetal resin is dissolved in a solvent, for example,composed of 70% toluene or xylene and ethyl alcohol. A solutioncontaining 25% to 40% solids is suitable for application to thelaminations.

. In some instances the bonding'composition may be modified by adding tothe polyvinyl acetal resin up to 20% of a phenol aldehyde heat hardeningresin. A cresylic acid-formaldehyde resin has been used with success, inthe temperature range of 100'C. to 130 C. The phenol aldehyde type whichwill contact, and thereafter etching the resin in the B stage is addedto the polyvinyl acetal in solution. A solution as prepared above may beapplied to the laminations for bonding purposes.

The phenol aldehyde resinous condensate addi'- tion may be modified withtung oil or oiticica oil up to 50% thereof` and introduced into thecom'- position containing polyvinyl acetal for use at operatingtemperatures below 125 C.

Where the electrical equipment is to be employed at the temperaturesbelow 125 C., the bonding composition may be modified to provide for agreater plasticity than obtained by the above compositions. Polyvinylacetate resin of an intermediate molecular weight, such, for example, asis sold on the market under the trade name of Vinylite (Ayat) has beenfound to give a satisfactory plasticizing effect in quantities o up to80% of the total resins.

A specific composition containing the above ingredients which has beenfound satisfactory for bonding the transformer cores shown in Fig. l ofthe drawing had the following proportions:

Per cent Polyvinyl acetal s 421/2 The above composition is dissolved ina solvent composed of approximately two parts of toluene or xylene andone part ethy1 alcohol to form a 25% to 40% solution. In some cases coaltar naphtha may be used in place of toluene or xylene.

Ths composition has produced bonds With strengths of 750 pounds persquare inch to 1290 pounds per square inch between laminations. It hasshown resistance to the chemical action of acids, dilute alkalis, waterwhen used in subsequent processing and to the oils or insulatingcompositions employed i-n transformers. The bonding composition will notow at temperatures of C. under operating conditions. Thus a ,stabletransformer core construction has been laminations. The particularresiny action of halogenated liquid trical insulating value betweenlaminations. The i subsequent treatment, however, is not alteredregardless of the form of coating applied.

I'he coated laminations are flashed at a high temperature for a shorttime in order to remove the solvent from the composition. Suitabletemperatures and times for this flashing are 450 C. for seconds, 350 C.for 40 to '70 seconds and 250 C. for 125 to 150 seconds. Temperaturesbelow 250 C. do not give as good results as treatment within the aboverange of temperatures.

After the flashing treatment the laminations are stacked into cores ofpredetermined size. Weighing may be employed to insure that the properamount of magnetic material has been put into the core. The stack isplaced within a clamping jig having adjustable portions whereby thestack may be consolidated under pressure to any selected thickness.However, the clamps at this stage are merely applied with a lightpressure.

The stack of laminations in the clamping jig is heat treated at 200 C.to 260 C. for 5 hours for a stack of the size of 4 inches by 6 inches by12 inches. During this time the resins react, particularly the phenolaldehyde resin which heat hardens.

At the end of theheat treating period, the stack of laminations with theclamps is removed from the ovens and the clamps are immediately adjustedto reduce the stack to the predetermined core size. 'I'he stack isallowed to cool in the clamps so adjusted to room temperature.

The stack of laminations after cooling may be ground and etched in orderto create smooth plane faces providing for accurate contact with otherstacks of laminations. This operation also removes the burrs. Theplurality of stacks of laminations may be then assembled into thetransformer il of Fig. 1 to give a transformer core construction whichis economical in cost and efficient in operation.

A modified construction of transformer core is disclosed in Fig. 2. Theconstructon of Fig. 2 employs for the entire core laminations ofspecially processed magnetic material, such as the Hipersil abovementioned. Due to the construction of Fig. 2 having the same crosssection throughout the transformer core, the specially processed siliconsteel may be used throughout.

The construction of Fig. 2 is further electrically eilicient in that thepath of the magnetic flux is so arranged that there are no substantialdiscontinuities or variations in the type of magnetic material it passesthrough. There are only two joints or M in each core unit.

The transformer 3l comprises two continuous wound cores severed intosimilar portions 32, $8 and 3l, The continuous cores are formed bywinding a continuous strip of magnetic material upon suitable formsuntil the desirable number been wound. The ends of are welded to thelamination immediately below.

The whole core is subjected to annealing to relieve internal strains dueto winding.

In order to assemble the primary coil Il and the secondary coil I2 aboutthe central leg of the transformer I0, it is most convenient to severthe wound cores into the parts Il, I2 and Il, Il. This severingoperation is facilitated by filling the spaces between laminations withthe bonding composition of the type disclosed hereinbefore.

The bonding composition solution may be applied by vacuum impregnatingthe annealed core in a tank. The coil is introduced into a tank, thetank is closed and vacuum applied to remove all the air. The bondingcomposition solution is run in from a storage receptacle until the coresor laminations are covered. Then evacuation is discontinued and nitrogengas is introduced and applied to the surface of the composition atatmospheric pressure in order to force the composition into the spacesbetween laminations. The laminations are allowed to soak for one-halfhour in order to impregnate them with the maximum amount of composition.Pressure can be applied to speed up the impregnation. 'I'he compositionis thereafter removed from the tank and the core is taken out.

In order to prevent the innermost laminations from warping duringsubsequent treahnent, it is desirable to place blocks, or Springs, 0rother spacing means within the window or open space within eachcontinuous core.

Thereafter, the impregnated coil is baked in ovens within a temperaturerange of 200 to 260 C. for four hours for a 35 pound core. 'Ihecomposition bonds the laminations well enough so that the cores may bereadily cut at the places indicated at 44 and 46 in the drawing withoutthe laminations breaking apart. Oil is used as a lubricant in thecutting operation due to the fact that the above process does not give afilling. The oil retards entrance of water, acid, etc., used insubsequent processing.

The joints are then ground and etched as indicated in the discussionwith respect to Fig. l. In this manner, perfectly flat and substantiallysmooth plane surfaces are obtained at joints "-46 and "-44 so that themagnetic flux has a minimum air gap loss.

excellent insulating of laminations have the strips of steel Thecomposition possesses properties at space factors of 97% to 85% with loweddy current loss between the laminations.

In some instances it has been found that the metal laminations can betreated with certain chemicals to cause an insulating film to formdirectly on the surface of the sheet metal. In this way a doubleinsulating effect is obtained. The nlm of insulating material formed bysuch treatment on the surface of thesheet material is an adherent scale,for example, a magnesium silicate fMgO-Si02 Brittle resins, for example,an all phenol-aldehyde type resin would give an exceedingly brittle andhard bonding composition which would tend to disrupt the scale from thesurface of the sheet metal under normal oper-- ating stresses and reducethe benet of such treatment. Tre bonding composition herein-- abovedisclosed is sufficiently plastic to avoid dis ruption of thisinsulating scale.

Since certain changes may be made in the1 above article and differentembodiments of the invention could be made without departing from thescope thereof, it is intended that all matters contained in the abovedescription or shown in the accompanying drawing shall be interpreted asillustrative and not in a limiting sense.

I claim as my invention:

1. A bonded core comprising laminations of magnetic material bonded by aheat-treated insulating composition between laminations, the bondingcomposition being composed oi from 20% to 80% of polyvinyl acetal, from0 to 80% of polyvinyl acetate for plasticizing the composition and fromsignicant amounts up to 20% oi a heat hardening phenol aldehyde typeresinous condensate to provide for a thermosetting agent duringheat-treatment to eiect consolidation of the laminations.

2. A bonded transformer core comprising lamnations of magnetic materialand a heat treated insulating composition between laminations to eifectconsolidation of the laminations, the composition including about 42.5%of polyvinyl acetal, about 42.5% polyvinyl acetate and about 15% ofcresylic acid and formaldehyde resinous condensate.

3. In a core structure, comprising in combination, a continuouslaminated wound core of a silicon magnetic steel with the direction ofthe easiest magnetization of the grains oriented in the direction of thewinding, the core being annealed after being wound. to' relieve internalstrains, and a heat-treated electrically insulating bonding compositionbetween laminations, the composition comprising polyvinyl acetal,polyvinyl acetate and a heat hardening phenol aldehyde type resinouscondensate, the composition having long chain molecules to provide fortoughness and elasticity and being characterized by resistance to the.chemical and physical action of acids, dilute alkali, water and oil.

4. In a core-structure comprising in combination, a continuous laminatedcore of a high silicon magnetic steel with the direction of the easiestmagnetization of the grains oriented in the direction of the winding,the core being annealed after being wound to relieve internal strains,and a heat-treated electrically insulating bonding composition betweenlaminations, the ,composition being composed of about 42.5% polyvinylacetate, about 42.5% polyvinyl acetal and about 15% of cresylicacid-formaldehyde condensate, the composition having long chainmolecules to provide for toughness and elasticity and beingcharacterized by resistance to the chemical and physical action ofacids, dilute alkali, water and oil.

JAMES G. FORD.

